The electronics industry is constantly driven to provide devices that are more powerful yet smaller, faster and less expensive than the previous generation. Over the years, the continual decrease of critical feature sizes necessitates the use of shorter and shorter exposure wavelengths, from visible (G-line, 436 nm) down to ultraviolet (UV) (I-line, 365 nm) to deep ultraviolet (DUV, 248 nm, KrF) to ultra-deep ultraviolet (193 nm, ArF) to 157 nm (F2), even extreme-UV (EUV, 13 nm) in semiconductor industry. 193 nm immersion lithography is rapidly emerging as a viable technology for 65 nm technology nodes and beyond. A good chemically amplified photoresist must meet all of the stringent requirements for microlithography at a given wavelength, such as transparent, chemically amplifiable, soluble in industry standard developer (e.g. tetramethyl ammonium hydroxide—TMAH), resistance to plasma etching, good adhesion to substrates, and exceptional thermal and mechanical properties for processing.
In order to sustain the plasma (RIE) etching process, a certain thickness of resist must be maintained (˜400 nm). Therefore, as feature size continues to decrease, the aspect ratios keep increasing (>5). One consequence is that the pattern collapses during the pattern transfer process. This has been one of the key issues when single layer resist is used for smaller features (i.e. 85 nm and beyond).
To cope with the pattern collapsing issue, multi-layer (ML) processes, such as tri-layer processes, are considered in which a thin single-layer (SL) resist is coated on top of a thin hardmask. The major challenge for the tri-layer process is to develop a hardmask layer which has not only high etch resistance, but also other matched optical properties both for the top-layer and the under-layer. There are also some other compatibility issues to be resolved. Furthermore, more steps and higher cost processes are expected.
Another type of multilayer process is the bi-layer (BL) process, in which a thin top image layer (˜150 nm), usually a silicon-containing resist, is applied to eliminate or minimize pattern collapse. The top image layer is cast on top of an underlayer, which is usually a high energy absorbing organic layer, such as anti-reflective coating (ARC). Overall, the bilayer process becomes more attractive, because it avoids many difficulties encountered in single and multi-layer approaches, and it is simple and a more cost effective process.
The silicon-containing BL resist provides good etch selectivity for anisotropic etch processes, such as reactive ion etching (RIE) using an oxygen containing plasma. In general, the higher the silicon content (wt %) in a silicon-containing resist the higher the etch resistance.
There are, however, many challenges for the development of silicon-containing resins useful in bilayer resist compositions for 193 nm or 157 nm photolithographic applications. First, incorporation of high silicon content (i.e., >15 wt %, needed for high etch resistance) into the polymer is difficult. Second, most silsesquioxane-based resins have low thermal stability (i.e., low Tg), and it is challenging to obtain high resolution, high sensitivity and a high process latitude for bilayer resist compositions. Third, outgassing of silicon-containing components during 193 nm exposure is one of the major concerns for silicon-containing polymers. Additionally, many resist formulation components, such as base quenchers and photo-acid generators (PAG), may interfere with the silsesquioxane resin structure, and therefore affect shelf-life.
As critical dimension (CD) continues to shrink, pattern collapse associated with conventional single-layer resists has become a serious issue due to their low etch resistance. There is a need to develop improved silicon-containing resins useful in bilayer resist compositions for 193 nm or 157 nm photolithographic applications, which have high etch resistance (high silicon content) with all the silicon incorporated into the polymer backbone to minimize silicon outgassing, as well as high thermal stability to improve process latitude. There is also a need for silicon-containing resist compositions that provide higher sensitivity and resolution for a larger process window. Additionally, there is a need for a silicon-containing resist composition that provides improved stability, and therefore extended resist shelf life.
Due to its unique structure and high content of Si—H bonds, hydrogen silsesquioxane (HSQ) is remarkably transparent at 193 nm and 157 nm. HSQ (commercialized by Dow Corning under the trade name FOx®) has been widely used as a spin-on low-k dielectric material, and already possesses certain characteristics required for a good photoresist, such as thin film quality and thermal and mechanical properties. It is also believed that in a base aqueous solution (like in the commonly used developer, tetra-methyl ammonium hydroxide (TMAH)), the Si—H bond is rapidly converted to a Si—OH moiety, which is base soluble. However it is very difficult, if not impossible, to directly incorporate any acid-labile functional groups onto the HSQ backbone to make HSQ useful as a photoresist.
WO 2005/007747, which is hereby incorporated by reference, describes HSQ-based resins of the general formula (HSiO3/2)a(RSiO3/2)b wherein R is an acid dissociable group, a has a value of 0.2 to 0.9, b has a value of 0.1 to 0.8 and 0.9≦a+b≦1.0. These HSQ-based resins are suitable as photoresists. However, these resins have shown instability over time; therefore limiting their application.